Paper products are used for a variety of purposes. Paper towels, facial tissues, toilet tissues, and the like are in constant use in modern industrialized societies. The large demand for such paper products has created a demand for improved versions of the products.
Generally, the papermaking process includes several steps. An aqueous dispersion of the papermaking fibers is formed into an embryonic web on a foraminous member, such as Fourdrinier wire, or a twin wire paper machine, where initial dewatering and fiber rearrangement occurs.
In a through-air-drying process, after the initial dewatering, the embryonic web is transported to a through-air-drying belt comprising an air pervious deflection member. The deflection member may comprise a patterned resinous framework having a plurality of deflection conduits through which air may flow under a differential pressure. The resinous framework is joined to and extends outwardly from a woven reinforcing structure. The papermaking fibers in the embryonic web are deflected into the deflection conduits, and water is removed through the deflection conduits to form an intermediate web. The intermediate web is then dried at the final drying stage, similarly to the conventional papermaking described above. At the final drying stage, the portion of the web registered with the resinous framework may be subjected to imprinting--to form a multi-region structure.
Through-air-drying paper webs are made as described in commonly assigned U.S. Pat. No. 4,514,345 issued to Johnson et al. on Apr. 30, 1985; U.S. Pat. No. 4,528,239 issued to Trokhan on Jul. 9, 1985; U.S. Pat. No. 4,529,480 issued to Trokhan on Jul. 16, 1985; U.S. Pat. No. 4,637,859 issued to Trokhan on Jan. 20, 1987; U.S. Pat. No. 5,334,289 issued to Trokhan et al on Aug. 2, 1994. The foregoing patents are incorporated herein by reference for the purpose of showing preferred constructions of patterned resinous framework and reinforcing structure type through-air-drying belts. Such belts have been used to produce commercially successful products such as Bounty paper towels and Charmin Ultra toilet tissue, both produced and sold by the instant assignee.
The woven reinforcing structure of the through-air-drying belts stabilizes and strengthens the resinous framework and reduces the permeability of the papermaking belt. Therefore, the reinforcing structure must have a suitable projected open area in order to allow the vacuum dewatering machinery employed in the papermaking process to adequately perform its function of removing water from the intermediate web, and to permit water removed from the web to pass through the papermaking belt. Therefore, the reinforcing structure should be highly permeable to fluids such as air and water.
At the same time, the reinforcing structure also serves an important function of supporting the cellulosic fibers, not allowing them be completely separated from each other or to be blown through the papermaking belt as a result of application of a vacuum pressure. These phenomena cause pin-sized holes, or pinholes, in the paper web. A large amount of pinholes reduces the quality of the paper web and may negatively affect the consumers' perception of the paper product. Therefore, the amount of fiber support provided by the reinforcing structure is of primary importance. Generally, a trade-off exists between the air permeability and fiber support of a papermaking belt. This trade-off is especially sensitive in through-air-drying belts which must have adequate open area for removing water from the web through the papermaking belt. Improvement in the fiber support of a belt by reducing its projected open area reduces the air permeability of the belt, or, vice versa, improvement in the air permeability of the belt by increasing its projected open area reduces the fiber support of the belt.
In order to mitigate the negative consequences of this trade-off between the air permeability and the fiber support of a papermaking belt, the early through-air-drying belts comprised a fine mesh reinforcing element. While such a fine mesh provided an acceptable fiber support, it was generally impractical because it did not provide necessary seam strength and resistance to the high temperatures encountered in papermaking.
A new generation of through-air-drying papermaking belts addressed these concerns. In these belts, a dual layer reinforcing structure significantly improved the seam strength and durability of the belts. In some dual layer reinforcing structures, a single cross-machine direction yarn system ties two machine direction yarn layers together, with the result of having vertically stacked machine direction yarns.
The use of a triple layer belt further improves a fiber support of the reinforcing structure. A triple layer belt comprises two completely independent woven layers, a top layer and a bottom layer, each having its own machine direction yarns interwoven with its own cross-machine direction yarns. The two independent woven elements are tied together with tie yarns.
Preferably, the top, or web-facing layer of the triple layer belt, has a finer mesh than the bottom, or machine-facing layer. The finer mesh provides a better fiber support and minimizes the amount of pinholes. The bottom layer utilizes coarser yarns to increase rigidity and improve seam strength. In a triple layer belt, the tie yarns may be specifically added to perform the function of linking the two independent layers together, without being present in either layer as a part of its inherent structure. Alternatively, the tie yarns may be the integral yarns forming the top and/or bottom layers of the reinforcing structure. In both cases, the tie yarns may be oriented in either the machine direction or the cross machine direction. Machine direction tie yarns are preferred because of the increased seam strength they provide.
European patent WO 91/14813 issued to Wright on Oct. 3, 1991 and assigned to Asten Group, Inc., describes a two-ply forming fabric having an upper paper carrying layer comprising twice as many cross-machine direction yarns as the lower, machine side, layer. A system of machine direction yarns interweaves in a selected pattern such that a zigzag effect is produced on the underside of the fabric to provide improved drainage.
U.S. Pat. No. 5,454,405 issued to Hawes on Oct. 3, 1995 and assigned to Albany International Corp. describes a triple-layer papermaking fabric having a system of top weft yarns and a system of bottom weft yarns interwoven with paired first and second warp yarns. The second warp yarns have relatively little crimp which increases stretch resistance in the fabric.
Although the use of double layer and triple layer reinforcing structures helps to balance the trade-off between the fiber support and the air permeability of the belt, the use of double and triple layer structures cannot, by itself, decouple these inherently interconnected characteristics.
Accordingly, it is an object of the present invention to provide an improved papermaking belt which substantially reduces the negative consequences of the trade-off between the air permeability and the fiber support of the belt. It is a purpose of the present invention to increase the available air permeability of the belt at constant fiber support, or to increase the available fiber support of the belt at constant air permeability.